A. Field of Invention
This invention relates to the field of lightweight, durable, molded frames for bicycles and other human-powered vehicles. More particularly, this invention relates to frames comprising a plurality of partial shells that have been molded from composite materials and that can be assembled to form a unitary monocoque frame that is strong, durable and lightweight. This invention also relates to a method for making the frame.
B. The Background Art
In the prior art, the structural frames of lightweight, human-powered vehicles such as bicycles, wheelchairs and the like have typically been made according to three methods: (1) Joined tubing, (2) inflation cured shells, and (3) truss structures.
1. Joined Tubing.
Joined tubing frames typically consist of metal tubing (such as steel, aluminum, titanium, etc.) that is either (1) brazed or adhesively bonded into lugs, or (2) miter-cut and welded tube-to-tube. Alternatively, joined tubing frames may be made of fiber-reinforced plastic (i.e. composite or "FRP") tubing that is adhesively bonded into lugs. Another common method of manufacturing frames from FRP tubing consists of forming composite lugs over pre-fabricated composite tubing.
Each of these methods of manufacturing frames from tubing is labor intensive, difficult to automate and therefore expensive. These methods also result in frames that are heavy because of the structural inefficiency of the joints of the frame. The joints are structurally inefficient because in order to achieve desired strength tolerances, tubing in the vicinity of the joints must be thick in comparison with the tubing at the center of the tube. Dual-thickness tubes such as these are referred to in the art as "double butted tubing" and add substantial weight to the resulting frame. Also, the lugs or joints into which the tubing is bonded or brazed must be relatively thick and hence heavy because the bending and torsional loads are inefficiently transferred to the lugs or joints from the non-integral tubing. The joined tubing frames made by these methods are unattractive to some users because of the angular, geometric appearance as opposed to more desirable smooth, flowing lines.
2. Inflation-Cured Shells.
Frames for lightweight vehicles may also be constructed by a method that eliminates tubing and joints, and results in a one-piece hollow shell, referred to in the art as a "monocoque" frame. The method uses composite materials consisting of a combination of structural fibers (such as carbon fiber and fiberglass) and thermoset resins (such as epoxy). When practicing the method, the fibers and resin are placed around an inflatable bladder (made from materials such as nylon film) or an expandable material (such as a heat-expandable foam). The assembly is placed in a cavity mold, and either the bladder is inflated or the expandable material naturally expands from the heat applied to cure the thermoset resin. This inflation or expansion presses the fiber and resin against the cavity mold walls conforming it to the shape of the mold during the curing of the resin. The mold is opened after the cure is complete, and the inflatable bladder or the expandable material permanently remains on the interior of the vehicle frame.
While this method eliminates joints and hence would seem to have the potential to reduce weight, it introduces other weight-increasing factors which offset the potential weight savings. The fibers are only strong in the fiber direction, and fibers are generally only available in single direction tape or two-direction (woven) cloth. In order for loads on the frame to be properly carried, the fiber must be plied in a number of layers, each in a different directional orientation. The end result is a significant thickness of the frame walls, especially in areas of the frame which are angled or bent (typically the same places on a frame which would be joints in a joined tubing frame), resulting in a frame which is heavy. Frames made by this method are typically brittle and subject to cracking from use and even shattering on impact, giving them a shorter product life compared to the invented frame. Also, frames made by this method are typically even more expensive to produce than joined-tubing frames because the process is very labor intensive and the amortized mold costs are very high since a mold can produce only one part every few hours. The frames made by this method can be very attractive, however, due to the flowing lines around the angles and bends.
2. Truss Structures.
Frames for lightweight vehicles may also be manufactured in a truss structure configuration. Generally a truss structure is comprised of a plurality of plate members that are each parallel to each other in a first direction, but may be of any thickness or shape in the second and third dimension. Typically the plates are parallel to each other in a first direction and thick in that direction, with different plates being thick and thin in second and third directions of varying orientation. This provides the frame with strength and rigidity in all three directions while resulting in a frame that can be injection molded and is lightweight.
Truss structure frames are inexpensive to manufacture, structurally sound, durable and lightweight, but users may have three basic objections to their use. First, the exposed edges of the thin truss structure plates are more subject to damage than are round-tubed or hollow-shelled structures. This may be a particular concern on vehicles that undergo use in extreme conditions, such as mountain bikes. Second, the appearance of the open truss structure is not attractive to some users because they prefer to smooth flowing lines of tubing. Third, the open truss structure can collect dirt, mud, debris, etc., and be difficult to clean because of limited access to the crevices of the truss structure. Round-tubed or hollow-shelled structures collect less dirt, mud, and debris and are easy to clean because dirt, mud, and debris can collect only on easily accessible areas.